Human TTK protein kinase (TTK), also known as tyrosine threonine kinase, dual specificity protein kinase TTK, Monopolar Spindle 1 (Mps1) and Phosphotyrosine-Picked Threonine Kinase (PYT), is a conserved multispecific kinase that is capable of phosphorylating serine, threonine and tyrosine residues when expressed in E. coli (Mills et al., J. Biol. Chem. 22(5): 16000-16006 (1992)). TTK mRNA is not expressed in the majority of physiologically normal tissues in human (Id). TTK mRNA is expressed in some rapidly proliferating tissues, such as testis and thymus, as well as in some tumors (for example, TTK mRNA was not expressed in renal cell carcinoma, was expressed in 50% of breast cancer samples, was expressed in testicular tumors and ovarian cancer samples) (Id). TTK is expressed in some cancer cell lines and tumors relative to normal counterparts (Id.; see also WO 02/068444 A1).
Agents which inhibit TTK have the potential to treat cancer. A number of potent TTK inhibitors are disclosed in International Patent Application Publication Nos. WO 2014/075168 and WO 2015/070349 (the entire teachings of which are incorporated herein by reference). One representative TTK inhibitor is N-cyclopropyl-4-(7-((((1s,3s)-3-hydroxy-3-methylcyclobutyl)methyl)amino)-5-(pyridin-3-yloxy)pyrazolo[1,5-a]pyrimidin-3-yl)-2-methylbenzamide, the structure of which is reproduced below as compound (I):

There is a need for salt forms of this compound that are crystalline and otherwise have physical properties that are amenable to large scale manufacture. There is also a need for pharmaceutical formulations in which this drug candidate is stable and is effectively delivered to the patient.